Electric power steering (EPS) systems generate a torque on the steering column of a vehicle. A common objective of the control methods for EPS is to reduce the effort of the driver in turning a steering wheel. See, e.g., U.S. Pat. No. 5,719,766, U.S. Pat. No. 5,894,205, U.S. Pat. No. 5,878,360 describing various configurations of the EPS systems and the operation of EPS systems to reduce the effort of the drivers in turning the steering wheel.
Steering system architectures have impact on a movement of the vehicle. Some methods describe uses of the EPS systems to improve the performance of cornering the vehicles and yaw stability control, see e.g., U.S. Pat. No. 4,951,199, U.S. 20110112716, U.S. Pat. No. 5,719,766. Similarly, U.S. Pat. No. 8,103,411 describes an over-steering control method. However, the application of those concepts is limited, because the EPS actuator is mechanically connected to the steering wheel and actuation of the EPS system affects the driver. Improper design or control of the EPS can be detrimental, and can range from a mild annoyance to the driver, e.g., a reduced “feel” for the road, to serious problems, such as lost sensitivity to the lateral response of the vehicle and perhaps a loss of maintaining the stability of the vehicle. In some cases, the improper design or control of the EPS can lead to threatening damages to the driver, such as excessive strain on driver arms leading to injuries.
Thus, the conventional EPS systems, in order to avoid improper impact of the EPS action on the driver, are designed as an extension of the driver arm and the value of torque applied by the EPS system to the steering column is uniquely determined by the current state of the steering mechanism.
As a consequence, conventional systems do not use EPS for different driving objectives, such as cornering performance and vehicle stability control. For example, conventional yaw stability control systems enforce vehicle stability control by generating a yaw moment through application of different torques to different wheels. For example, the system described in U.S. Pat. No. 7,966,113 B2 uses brakes to achieve different torques. That system can be effective for rapid stability recovery, but can cause discomfort to the driver due to sudden unexpected rotational and longitudinal decelerations. In addition, the braking actuation mechanism is not precise, so that the engagement of the system is delayed as much as possible and used only for stability recovery, and not for precise stability control.
Vehicle cornering performance can also be improved by using variable steering gears, also known as active steering mechanisms that allow changing the relation between the steering wheel angle and the wheels. However, that approach requires specific actuators, which are costly and have the following disadvantages: the driver may lose the “feel for the road”, i.e., the driver does not feel the alignment torque that is the indicator for stability losses; and the steering always moves in the direction of least resistance, hence if the driver releases the steering wheel, the actuator turns the steering columns and not the wheels.
Accordingly, there is a need to use the EPS systems to assist in all of the above situations and many others, while avoiding the improper effect of the EPS action on the driver.